Better support for pytest.param in parametrize_plus

[smarie]

Fixes #79
Fixes #86

[codecov-commenter]

Codecov Report

Merging #89 into master will increase coverage by 0.39%.
The diff coverage is 93.50%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
+ Coverage   88.08%   88.48%   +0.39%     
==========================================
  Files          60       67       +7     
  Lines        2544     2787     +243     
==========================================
+ Hits         2241     2466     +225     
- Misses        303      321      +18     

Impacted Files	Coverage Δ
pytest_cases/tests/issues/test_issue_pytest_70.py	100.00% <ø> (ø)
pytest_cases/common.py	86.71% <85.71%> (-1.17%)	⬇️
pytest_cases/mini_six.py	92.30% <92.30%> (ø)
pytest_cases/main_fixtures.py	88.56% <92.76%> (+0.69%)	⬆️
...t_cases/tests/fixtures/test_fixture_ref_custom2.py	92.85% <92.85%> (ø)
pytest_cases/main_params.py	82.73% <100.00%> (ø)
pytest_cases/plugin.py	82.97% <100.00%> (+0.03%)	⬆️
...st_cases/tests/fixtures/test_fixture_ref_basic1.py	100.00% <100.00%> (ø)
...st_cases/tests/fixtures/test_fixture_ref_basic2.py	100.00% <100.00%> (ø)
...s/tests/fixtures/test_fixture_ref_basic3_tuples.py	100.00% <100.00%> (ø)
... and 15 more

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